NGU - BULL 427. 1995 Arne J. Reite 19

Deglaciation of the Trondheimsfjord area, Central Norway

ARNE J. REITE

Arne J. Beite, Norges geologiske undersokelse, P.O.Box 3006-Lade. N-7002 Trondheim, Norway.

Introduction Trondheimsfjorden, terminal are found The purpose of this contribution is to provide an in some of the and locally in lowland areas, account of our present knowledge of Late but cannot be traced into higher land (Fig. 1). Weichselian deglaciation of the Trondheimsfjord These moraines were deposited by an unstable area. The results are based on two decades of ice front caused by calving and not by a climatic mapping of Quaternary deposits in this part of deterioration (Kjenstad & Sollid 1982, Sollid & the country. During the Weichselian maximum, Reite 1983). They are not strictly synchronous as Central Norway was completely covered by an the retreat by calving was highly depen­ inland ice that moved in a WNW direction. Later, dent on water depth, exposure to wave activity the glacier flow became more dependent on the and topographical conditions (Reite 1994). topography (Reite 1994). During the deglaciati­ Generally, these terminal moraines, radiocarbon on, the Trondheimsfjord area was deeply sub­ dated to c. 12,500 years S.P., are strongly rewor­ merged by the sea, and the fjords reached far up ked in the tidal zone during the shoreline displa­ into the present valleys. cement (Fig. 1& 2). Deglaciation of the coastal areas Deglaciation of the outer parts of the In the coastal areas to the north and south of fjords After a readjustment of the glacier gradient, the calving continued into Trondheimsfjorden and other fjords. This process was accelerated by the great depth of the central part of most of these fjords compared to the much shallower thres­ holds at their mouths. Local ice caps with slight or no connection to the inland ice remained in the mountainous areas along the fjords (Fig. 1), and small ice-marginal deposits were formed in front of outlet (Reite 1994). As the wes­ tern part of the area was deglaciated several hundred years before the central part, these ice caps are not synchronous. They were situated far below the glaciation limit, and probably exi­ sted for less than 100 years. Deglaciation of the central part of the Trondheimfjord area The calving halted temporarily where the ice front reached shallower water in the fjords and lower parts of the present valleys that were sub­ merged during the deglaciation. Glaciofluvial sediments accumulated along the ice margin, especially in the major valleys. High salinity led to f1 occulation of suspended material in silt and

o 10 20 30 km size fractions, and sedimentation occurred ~---- mostly within a few km from where the suspen­ ded material reached the fjords (Fig. 3). In Fig. 1. Ice-marginal deposits in the Trondheimsfjord area. A - A Ice-marginal deposits in the coastal areas (c. 12,500 years Trondheimsfjorden, the calving during the BP.). B • B The Tautra ice-marginal deposits (early Younger AIIer0d chronozone continued to a position at Dryas). C • C The Hoklingen ice-marginal deposits (late Younger Dryas). The shaded areas represent local ice caps least 15 km to the east of the Tautra ice-marginal left after calving in the fjords and the submerged parts of the zone (Figs. 1 & 2). Few distinct terminal moral- present valleys . Modified from Reite (1994). 20 Arne J. Reite NGU · BULL 427. 1995

E l 0ALANDET TAUTRA HOKU NGEN VUKU ~ ~

to500 J YOUNGER ~.C APS .. ? -. I ORYAS c-.. DEGLA CIATED ~_ ,;,: ? __ :,,;:'... INlAND ICE

Fig. 2. Time-distance diagram for the AllEROO deglaciation of the Trondheimsfjord l OCAL ICE CAPS j area.Local ice caps left after fast 12.000 calving in the fjords and submerged 6 0ll.ING valleys are also indicated. Modified I from Reite (1994). nes were deposited during this part of the degla­ zone compared to that of late Allerod, shows that ciation. According to radiocarbon datings of mol­ there was a considerable increase in the thick­ luscs and gyttja , the deglaciation of the outer part ness of the inland ice during this advance. of the Trondheimsfjord area took place during Distinct shorelines in solid bedrock are found just the Older Dryas chronozon e. The inland ice outside the ice margin during the Tautra glacial receded from the central part of the in the advance but not elsewhere in the middle of the Allerod chronozone and reached Trondheimsfjord area, indicating a colder climate its easternmost position before the Younger than for the rest of the deglaciation. Radiocarbon Dryas glacier advance at the transition datings of molluscs, whale bones and gyttja indi­ Allerad/Younqer Dryas (Reite et al. 1982). cate that the Tautra ice-marginal deposits were formed 10,800- 10,500 years S.P. Glacier advance to the Tautra ice-marginal deposits Deglaciation after the Tautra event The deglaciation during the Altered chronozone At the middle of the Younger Dryas chronozone was followed by a marked glacial readvance the recession of the inland ice continued to posi­ caused by the Younger Dryas cooling. During tions 20-50 km to the east of the Tautra ice-mar­ this event, Allerod sediments were overrun by ginal deposits (Fig. 1). This recession halted the inland ice and the Tautra ice-marginal depo­ temporarily at rock thresho lds and spurs , sits were form ed (Figs. 1, 2 & 3). They can be and ice-conta ct deltas, terraces, sandurs traced almost continuously from Melhus to the and a few term inal moraines were formed. Fast northern part of the Fosen Peninsula. In higher calving in the fjords and submerged valleys left areas these ice-marginal deposits appear to be local ice caps in higher areas. Terminal moraines push moraines; in lowland areas glaciofluvial and glaciofluvial ice-marginal deposits were for­ sediments also occur , commonly with lenses of med in front of outlet glaciers from these ice in their proximal part, indicating oscillations of caps , that had little or no connect ion to the inland the ice front. The position of the inland ice during ice. The climate during middle Younger Dryas, the glacial advance to the Tautra ice-marginal judged from the fast retreat of the inland ice and the occurrence of large glaciofluvial deposits,

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" :. .... - ""'---" _._._. -- - - . ~ -.; --< : .< ~ .- . .. ..-.-.""'--....-.....-:: ~_~- _=.. ""' ...... Fig. 3. Stratigraphy (simplified) of are· as below the upper marine limit in the ~ Marine c lays older than AJ l er{~ [=-4 HoIocene clays Trondheimsfjord area (see Figs. 1 & 2). Thin till layers from glacial read­ '- - -I AJlerod Cla ys ~ ~e · marginaJ deposits vances are not shown. Most marine

~ clays w e re uepostteo within a few km ---=- early/ M id d le You ng er Dryas Cla ys Bed rock from where the meltwater streams reached the sea.Holocene clays 1=-:'::1 late Younge r Oryas clays Sea- level were deposited during the shoreline displacement. NGU - BULL 427, 1995 Arne J. Reite 21 may have been warmer than during the Tautra ments also occur. In areas deglaciated after the and Hoklingen events. However, the climate was Tautra and before the Hoklingen glacial advan­ no doubt colder than during the Allerod chrono­ ces, large glaciofluvial deposits occur, indicating zone. Due to a colder climate, ice caps formed a warmer climate than during the Tautra and during the deglaciation shortly after the Tautra Hoklingen events. This is also indicated by the event may have existed for a longer time than marked recession before the glacial advance those from the Allerod retreat, perhaps for 100­ towards the Hoklingen ice-marginal deposits. At 200 years. the transition Younger Dryas/Prebor eal a mar­ ked climatic amelioration took place that led to a Glacier advance to the Hoklingen fast thinning of the inland ice. ice-marginal deposits The recession from the Tautra ice-marginal References Kjenstad, K. & SoUid, J . L. 1982: Isavsmeltningskronologi i deposits was followed by a glacial advance of at Trondhelrnsfjordomradet. Glacialdynamiske prinsipper. least 10 km to the Hoklingen ice-marginal depo­ Nor. Geogr. Tidsskr. 36 , 152-162. sits (Figs. 1 & 2). The inland ice increased in Reite, A. J. 1994: Weichselian and Holocene geo logy 01 Sor­ thickness and advanced across sediments depo­ Trondelag and adjacent parts 01 Nord-Trondelag cou nty, Central Norway. Nor. geol. unders. Bull. 426, 1-30. sited during the recession (Fig. 3). In higher land, Reite, A. J., Seines, H. &Sveian, H. 1982: A proposed deglaci­ the Hoklingen ice-marginal zone consists of dis­ ation chronology lo r the Trondheimsljord area, central tinct terminal moraines that appear to be push Norway. Nor. geol. unders. 3 73, 75-84 . moraines . In lowland areas, both terminal morai­ SoUid, J . L. & Reite, A. J. 1983: The last glaciation and deglaci­ ation 01Central Norway (41-66). In Ehlers, J. (ed.): Glacial nes and glaciofluvial ice-marginal deposits deposits in North-West Europe. A. A. Balkema, Rotterdam, occur. Radiocarbon datings of molluscs and peat 500 pp. strongly indicate that the Hoklingen ice-marginal Sveian , H. 1989: Stiklestad. Kvartrergeologisk kart 1722 IV­ deposits were formed during late Younger Dryas, M. 1:50 000. Beskrivelse. Nor. geol. unders. Skr. 89, 1-54 . c. 10,300 years S.P. (Reite et al. 1982, Sveian 1989). Deglaciation after the Hoklingen event During the retreat of the inland ice from the Hoklingen ice-marginal zone, the ice front was temporarily halted at favourable positions in the valleys and large glaciofluvial deltas, kame terra­ ces, and sandurs were deposited. Some terminal moraines were also formed ; the most distinct of these are the Vuku ice-marginal depo­ sits, dated to early Preboreal (Reite et al. 1982, Sveian 1989). This part of the deglaciation took place during a marked climatic amelioration, and is characterised by a fast thinning of the inland ice, leaving the last remnants of ice in depressi­ ons in the terrain. Climatic changes during the deglaci­ ation of the Trondheimsfjord area Areas deglaciated during Older Dryas and the first half of the Allerod chronozone are characte ­ rised by few and small glaciofluv ial deposits. During the second half of the Allerod chronozo­ ne, glaciofluvial deposits were formed in the lower parts of most valleys; this may indicate a climatic amelioration. The Tautra and Hoklingen ice-marginal deposits, caused by the marked Younger Dryas cooling, consist of terminal moraines deposited by marked glacial readvan­ ces, but in the major valleys glaciofluvial sedi-